In recent years, the so-called nonpolluting automobiles have been developed, and a part of them has been put to practical use with the aim of preventing the environmental deterioration caused by the exhaust gas of automobiles. For example, an automobile operating by means of a natural gas in which methane is the main component as the fuel has been authorized as a low pollution automobile, and about one million units of the automobiles have been actually used already all over the world.
Moreover, it is thought that a fuel cell powered automobile using hydrogen as its fuel will substitute for most of the current gasoline-driven automobiles in the future. There is a possibility that the natural gas and hydrogen that are used here are compressed natural gas and compressed hydrogen, respectively, and it is thought that the pressures will be ultrahigh pressures, such as 20 MPa to 25 MPa for the natural gas and 35 MPa to 70 MPa for hydrogen. Consequently, a tank (pressure-resistant container) having a pressure-resistant performance is needed, and a high pressure tank made of steel has been used conventionally.
When natural gas and hydrogen are used as the fuels of automobiles, it is preferred that the tanks to be mounted are light weighted from the point of view of the improvement of engine performance, the improvement of fuel consumption and the like. A tank made of an aluminum liner which is reinforced with carbon fibers has been used as an alternative for the steel tank. However, a tank using a resin liner in which the entire tank is made of resin has been developed in order to achieve further lightweight (for example, Patent Document 1).
The tank made of resin liner is a gas tank composed of a resin inner shell (inner wall) having a gas barrier property and a fiber reinforced plastic (FRP) outer shell (outer wall) having a pressure resistance property wherein the outer shell covers the inner shell, and is lighter than a metal tank because the tank made of resin liner is mainly made of resin.
This sort of gas tank is provided with a nozzle attaching mouth ring in order to attach a nozzle for filling a gas into the tank and for taking the gas out from the tank.
The mouth ring portion is generally joined with the inner shell integrally. The mouth ring portion for screwing the nozzle or the like is generally made of a metal, and the inner shell is made of a material (the above mentioned resin, FRP or a light metal) different from that of the mouth ring portion from the point of view of the weight saving or the simplification of the manufacturing process. Because the mouth ring portion and the inner shell which are made of different materials from each other are integrally joined, the sealing property of the joined part or the interface part of the inner shell and the mouth ring portion becomes important.
In particular, because the high pressure gas up to about 70 MPa is filled in the gas tank which is called the above mentioned high pressure natural gas tank or high pressure hydrogen gas tank for an automobile, an extremely high gas sealing property is required. The conventional gas tank yet has insufficient gas sealing property of the joined part or the interface part of the inner shell and the mouth ring portion.
Patent Documents 2 to 5 propose pressure-resistant containers having improved gas sealing property of the mouth ring portion in the high pressure gas tanks made of resin.
The pressure-resistant container described in Patent Document 2 has the structure of enhancing the gas sealing property of the mouth ring portion by adopting the structure of receiving a disk-like flange part of the mouth ring portion with upper and lower lips at the end part of the inner shell.
However, in the pressure-resistant container described in Patent Document 2, the end part of the mouth ring is exposed to its internal gas, and the internal pressure of the gas is directly applied to the end part of the mouth ring portion. Consequently, even when gas leakage does not occur just after the manufacturing of the pressure-resistant container, creeping occurs in the resin of the inner shell to shrink and a gap is generated at an interface of the inner shell and the mouth ring while using the container for a long time. Thus, there is a possibility that gas leakage might occur from the gap at the interface.
Moreover, in the pressure-resistant container described in Patent Document 3, locking grooves are provided at the upper and lower surfaces of a disk-like flange part of the mouth ring, and a tab which fits with the upper and lower locking groove is formed at the end part of the inner shell to join the upper and lower locking grooves. Thereby, the pressure-resistant container is structured so as to oppose the force which tears off both the lips with the locking groove, which force is a shearing force operates on both the lip parts joined to the locking groove at the upper and lower surfaces of the disk-like flange when the inner shell swells at the time of the swelling of the container by an internal pressure when a high pressure gas is kept in the pressure-resistant container.
However, in the pressure-resistant container described in Patent Document 3, the end part of the mouth ring is also exposed to the internal gas, and the internal pressure of the gas is applied directly to the end part of the mouth ring. Consequently, there is possibility that gas leakage might occur by the same reason as described above.
Furthermore, the pressure-resistant container described in Patent Document 4 aims to enhance the gas sealing property by adopting the following structure: the mouth ring intervenes at the inner side of the opening part of the inner shell manufactured by the blow molding; a sealing ring made of rubber or the like is fitted to the shoulder part of the inner shell; the sealing ring is pressed by a pressing member made of a metal or the like; and filament winding molding is performed thereover. Even when the pressing member of the sealing ring is made of a metal, the fitting part of the sealing ring is only structured with the resin-made inner shell. Therefore, even when a gas leakage does not occur right after the manufacturing of the pressure-resistant container, the plastic of the inner shell creeps and the inner shell shrinks while the container is being used for a long time. Thus, there is a possibility that gas leakage might occur in the pressure-resistant container similarly as the pressure-resistant containers described in Patent Documents 2 and 3. Even when it is tempted to increase the number of sealing ring in response to the shrinking of the inner shell, the product container is needed to be disassembled, and further in the case of exchanging a deteriorated sealing, the product container is also needed to be disassembled, and it is not only troublesome but also uneconomical.
As a pressure-resistant container to solve the problem which is common to the above pressure-resistant containers described in Patent Documents 2 to 4, there is the pressure-resistant container described in Patent Document 5.
In the container described in Patent Document 5, a cylindrical neck part, a tabular extending part extending outward and a bent part bent to the inner shell side are successively connected to the pole portion of the inner shell, and the cross section of the end part of the inner shell is bended to the inner shell side to be embedded in the mouth ring and an O-ring is jointly used between the pressing part and the extending part. Thereby, the end part of the mouth ring is prevented from being exposed to the internal gas and the internal pressure of the gas is prevented from being applied directly to the end part of the mouth ring.
However, in this method, although one side is the pressing member made of metal which can secure the surface accuracy relatively easily, the other side is made of plastic which cannot secure the surface accuracy. Consequently, this structure lacks the reliability of sealing. Moreover, it is needed to use rotational molding which takes long time for the molding of the plastic liner of the inner shell in order to bend the cross section of the end part of the inner shell to the inner shell side to be embedded into the mouth ring, and the lowering of the cost of the inner shell is prevented. Moreover, in order to support the bent part of the inner shell with the end part of the mouth ring in the inner side of the tank, the end part is formed so as to project toward the inner side. The projecting part causes no problem in the case of a relatively low pressure tank. However, the bending strength of the mouth ring at the projecting part by the internal pressure of the tank becomes severe in an ultra high pressure tank, such as 70 MPa, and there is a possibility that the breaking of the mouth ring occurs in a repetition cycle.
Patent Document 1: Japanese Patent Publication No. Hei 5-88665
Patent Document 2: Japanese Patent Application Laid-Open Publication No. Hei 6-42698
Patent Document 3: Japanese Patent Application Laid-Open Publication No. Hei 6-137433
Patent Document 4: Japanese Patent Application Laid-Open Publication No. Hei 8-219387
Patent Document 5: Japanese Patent Application Laid-Open Publication No. Hei 10-332085